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Date: Wed, 27 Mar 2013 07:43:52 -0400
From: Peter Hurley <peter@...leysoftware.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jslaby@...e.cz>
Cc: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
Min Zhang <mzhang@...sta.com>, Ilya Zykov <linux@...k.ru>,
Peter Hurley <peter@...leysoftware.com>
Subject: [PATCH v2 02/18] tty: Make ldisc input flow control concurrency-friendly
Although line discipline receiving is single-producer/single-consumer,
using tty->receive_room to manage flow control creates unnecessary
critical regions requiring additional lock use.
Instead, introduce the optional .receive_room() ldisc method which
returns the maximum # of bytes the .receive_buf() ldisc method can
accept. Serialization is guaranteed by the caller.
In turn, the line discipline should schedule the buffer work item
whenever space becomes available; ie., when there is room to receive
data and receive_room() previously returned 0 (the buffer work
item stops processing if receive_room() is 0).
Add n_tty_receive_room() as the receive_room() method for N_TTY
and remove tty->receive_room references in N_TTY.
Line disciplines not using input flow control can continue to set
tty->receive_room to a fixed value.
Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
drivers/tty/n_tty.c | 68 ++++++++++++++++++++++++++++++-----------------
drivers/tty/tty_buffer.c | 3 +++
include/linux/tty_ldisc.h | 8 ++++++
3 files changed, 54 insertions(+), 25 deletions(-)
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 9b7f571..a185aff 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -74,11 +74,17 @@
#define ECHO_OP_SET_CANON_COL 0x81
#define ECHO_OP_ERASE_TAB 0x82
+/* Bit values for flags field
+ */
+#define NO_ROOM 0
+
struct n_tty_data {
unsigned int column;
unsigned long overrun_time;
int num_overrun;
+ unsigned long flags;
+
unsigned char lnext:1, erasing:1, raw:1, real_raw:1, icanon:1;
unsigned char echo_overrun:1;
@@ -114,25 +120,10 @@ static inline int tty_put_user(struct tty_struct *tty, unsigned char x,
return put_user(x, ptr);
}
-/**
- * n_tty_set_room - receive space
- * @tty: terminal
- *
- * Updates tty->receive_room to reflect the currently available space
- * in the input buffer, and re-schedules the flip buffer work if space
- * just became available.
- *
- * Locks: Concurrent update is protected with read_lock
- */
-
-static int set_room(struct tty_struct *tty)
+static ssize_t receive_room(struct tty_struct *tty)
{
struct n_tty_data *ldata = tty->disc_data;
int left;
- int old_left;
- unsigned long flags;
-
- raw_spin_lock_irqsave(&ldata->read_lock, flags);
if (I_PARMRK(tty)) {
/* Multiply read_cnt by 3, since each byte might take up to
@@ -150,18 +141,25 @@ static int set_room(struct tty_struct *tty)
*/
if (left <= 0)
left = ldata->icanon && !ldata->canon_data;
- old_left = tty->receive_room;
- tty->receive_room = left;
-
- raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
- return left && !old_left;
+ return left;
}
+/**
+ * n_tty_set_room - receive space
+ * @tty: terminal
+ *
+ * Re-schedules the flip buffer work if space just became available.
+ *
+ * Locks: Concurrent update is protected with read_lock
+ */
+
static void n_tty_set_room(struct tty_struct *tty)
{
+ struct n_tty_data *ldata = tty->disc_data;
+
/* Did this open up the receive buffer? We may need to flip */
- if (set_room(tty)) {
+ if (receive_room(tty) && test_and_clear_bit(NO_ROOM, &ldata->flags)) {
WARN_RATELIMIT(tty->port->itty == NULL,
"scheduling with invalid itty\n");
/* see if ldisc has been killed - if so, this means that
@@ -174,6 +172,27 @@ static void n_tty_set_room(struct tty_struct *tty)
}
}
+/**
+ * n_tty_receive_room - receive space
+ * @tty: terminal
+ *
+ * Called by flush_to_ldisc() to determine the currently
+ * available space in the input buffer.
+ *
+ * Locks: Concurrent update is protected with read_lock
+ */
+
+static ssize_t n_tty_receive_room(struct tty_struct *tty)
+{
+ struct n_tty_data *ldata = tty->disc_data;
+
+ ssize_t room = receive_room(tty);
+ if (!room)
+ __set_bit(NO_ROOM, &ldata->flags);
+
+ return room;
+}
+
static void put_tty_queue_nolock(unsigned char c, struct n_tty_data *ldata)
{
if (ldata->read_cnt < N_TTY_BUF_SIZE) {
@@ -1465,8 +1484,6 @@ static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
tty->ops->flush_chars(tty);
}
- set_room(tty);
-
if ((!ldata->icanon && (ldata->read_cnt >= ldata->minimum_to_wake)) ||
L_EXTPROC(tty)) {
kill_fasync(&tty->fasync, SIGIO, POLL_IN);
@@ -1481,7 +1498,7 @@ static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
*/
while (1) {
tty_set_flow_change(tty, TTY_THROTTLE_SAFE);
- if (tty->receive_room >= TTY_THRESHOLD_THROTTLE)
+ if (receive_room(tty) >= TTY_THRESHOLD_THROTTLE)
break;
if (!tty_throttle_safe(tty))
break;
@@ -2201,6 +2218,7 @@ struct tty_ldisc_ops tty_ldisc_N_TTY = {
.receive_buf = n_tty_receive_buf,
.write_wakeup = n_tty_write_wakeup,
.fasync = n_tty_fasync,
+ .receive_room = n_tty_receive_room,
};
/**
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index a42a028..f294631 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -449,6 +449,9 @@ static void flush_to_ldisc(struct work_struct *work)
tty_buffer_free(port, head);
continue;
}
+
+ if (disc->ops->receive_room)
+ tty->receive_room = disc->ops->receive_room(tty);
if (!tty->receive_room)
break;
if (count > tty->receive_room)
diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index 23bdd9d..6a8cb18 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -109,6 +109,13 @@
*
* Tells the discipline that the DCD pin has changed its status.
* Used exclusively by the N_PPS (Pulse-Per-Second) line discipline.
+ *
+ * ssize_t (*receive_room)(struct tty_struct *tty)
+ *
+ * If defined, returns the current # of bytes the receive_buf()
+ * method can accept. If not defined, this value is determined by
+ * the tty->receive_room value (which may be static if the line
+ * discipline does not perform flow control).
*/
#include <linux/fs.h>
@@ -195,6 +202,7 @@ struct tty_ldisc_ops {
void (*write_wakeup)(struct tty_struct *);
void (*dcd_change)(struct tty_struct *, unsigned int);
void (*fasync)(struct tty_struct *tty, int on);
+ ssize_t (*receive_room)(struct tty_struct *tty);
struct module *owner;
--
1.8.1.2
--
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